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North Sea gas leak venting from newly disturbed source

By Andy Coghlan and Michael Marshall

A major methane gas leak is under way at the Elgin wellhead in the North Sea, 240 kilometres off Aberdeen, UK. The leak started on 25 March, but according to sources at Total, the company operating the well, the gas is not coming from the gas reservoir itself, but from a newly disturbed source in the rock above.

Many questions remain. Total says that until it works out the capacity of the source and the rate at which methane and gas condensate are leaking into the environment, it is impossible to say either how much gas will be released or how long it will take to block it, despite some reports putting it at six months.

“We’ve got geologists working on the productivity of the horizon [reservoir] the leak is coming out of,” Andrew Hogg, a spokesman for Total, told New Scientist. “We must do some modelling to find out the rate.”

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Although the main reservoir itself at the base of the drill shaft is safely closed off, Hogg says, the gas from the secondary source in chalk above it is escaping by leaking into the shaft containing the drilling tubes that lead down to the main gas reservoir.

Sealing the leak

One option to halt the leak would be to pump heavy mud down the shaft to stop the gas, but that would require access to the platform, which is currently too dangerous because of the risk of fire.

The optimal solution is a self-sealing event, in which the pressure dips as gas vents, so the leak effectively plugs itself. But the likelihood of this will depend on how much gas is in the chalk, and how chambers are connected by fractures and channels. Total has called in well-control specialists from the US.

The platform was drilling for sour gas&colon; natural gas polluted with hydrogen sulphide and carbon dioxide, which 20 years ago would have been too expensive to extract. “It’s gas we started using as a last resort,” says Simon Boxall of the National Oceanography Centre in Southampton, UK. The gas is purified on the platform itself, before being taken to the mainland.

Ecosystem threat

The major threat to the local ecosystem is the hydrogen sulphide, which is toxic to virtually all animal life. Because the leak is below the water’s surface, the hydrogen sulphide is bubbling through the sea water. This is the worst-case scenario, says Boxall, because it could lead to mass animal and plant deaths. Boxall says Total needs to monitor the water quality to see if this is happening.

Much of the methane in the water will be consumed by microorganisms and converted to carbon dioxide. This will make the water slightly more acidic, but the effect will be short-lived and localised, and therefore should not cause too much harm to marine life.

The volatile mix of gases means there is a significant risk of an explosion. As a result, Total have shut down the power supply on the rig, which otherwise might throw a spark. Hogg told New Scientist that the leaking methane posed no safety risks to neighbouring rigs, the nearest of which is the Shearwater rig owned by Shell, about 7 kilometres away.

All three of the escaping gases are greenhouse gases, but unless the leak carries on for weeks or months, the effect on the climate is likely to be small. “The impact on climate change is not going to worry anyone,” says Boxall.

“With wind and atmospheric conditions, the gas disperses and the petrol-like condensate on the surface will disperse quite quickly too,” Hogg said.

In an earlier version of this story Simon Boxall compared the effect of hydrogen sulphide to Agent Orange. He has since pointed out that Agent Orange primarily affects plant life whereas hydrogen sulphide primarily affects animals, so for the sake of clarity we have removed this reference.